The present invention relates to a method and apparatus for using speckle-shearing interferometry to analyze the deformation of an object when a stress is applied to it. More specifically to locate subsurface defects in pneumatic tires such as voids, separations, or nonuniformities.
Nondestructive means of analyzing the deformation of an object when a stress is applied thereto are useful in quality control and engineering studies. As used herein, "deformation" refers to the displacement of points on the surface of an object relative to one another resulting from a stress imposed on the object, as opposed to overall, uniform displacements resulting from the stress.
There are several widely recognized ways of analyzing surface deformation including at least: point-by-point mapping of the object surface before and after the imposition of a stress; holographic interferometric techniques; birefringent-coating techniques; and speckle-shearing interferometric techniques. U.S. Pat. No. 4,139,302, issued to Hung et al, teaches a method and apparatus for analyzing the strain on an object resulting from the application of a stress employing speckle-shearing interferometric techniques. When a surface is illuminated by a source of coherent light, such as a laser, it has a grainy appearance which is known as speckle effect. This speckling, of light and dark points, is due to random interference of light reflectively scattered from different depths on the object's surface. These bright speckles are small regions where the light reflected from the object is of the same phase, giving the speckle its bright appearance from mutual reinforcement. As used herein, "shearing" refers to focusing light reflected from an object through an optical device to form two focused images of the object slightly displaced with respect to one another. By interfering two speckle-sheared patterns on a photographic media, corresponding to the undeformed and deformed states of the surface of an object, a fringe pattern is produced that contours the displacement gradient of the surface.